The present invention generally relates to methods of producing materials and articles with nanocrystalline and ultra-fine grained (UFG) (together called nanostructured) microstructures, and more particularly to directly producing monolithic bodies having nanostructured microstructures through a machining process.
Significant benefits can be gained by deforming metals and metal alloys through the application of very large plastic strains. Principal among these are microstructure refinement and enhanced mechanical and physical properties. Of particular current interest is the use of “severe” plastic deformation (SPD) to produce bulk solids with nanostructured microstructures, i.e., ultra-fine grained (UFG) microstructures, especially nanocrystalline structures (NS) characterized by their atoms arranged in crystals with a nominal dimension of less than one micrometer, particularly less than 500 nm. Nanocrystalline solids have become of interest because they appear to exhibit improved strength, ductility, formability and resistance to crack propagation compared to microcrystalline materials, and possess interesting chemical, optical, magnetic and electrical properties. Nanocrystalline solids also appear to respond to radiation and mechanical stress quite differently than microcrystalline materials (comprising crystals with a nominal dimension of one micrometer to less than one millimeter), and their response can be varied by changing the crystal size. Materials made by consolidating nanocrystalline powders have also been shown to have enhanced attributes not typically found in conventional materials. As a result, nanocrystalline materials are believed to have significant potential for use in industrial applications, provided they can be manufactured in a cost-effective manner.
Multi-stage deformation processing is one of the most widely used experimental SPD techniques for studying microstructural changes produced by very large strain deformation. Notable examples include rolling, drawing, high-pressure torsion (HPT), and equal channel angular extrusion (ECAE) processes. In a multi-stage deformation process, very large plastic strains (shear strains of four or more) are imposed in a specimen by the cumulative application of deformation in multiple discrete stages, the effective strain in each stage of deformation being on the order of two. The formation of micro- and nanocrystalline structures has been demonstrated in a variety of ductile metals and alloys using multi-stage deformation processing. However, there are significant limitations and disadvantages with this processing technique. A significant limitation is the inability to induce large strains in high-strength materials, such as nickel-based high-temperature alloys and tool steels. Other limitations include the difficulty of imposing strains of much greater than one and inability to impose strains of much greater than two in a single stage of deformation, the considerable uncertainty of the deformation field, and the minimal control over the important variables of the deformation field—such as strain, temperature, strain rate and phase transformations—that are expected to have a major influence on the evolution of microstructure and material properties.
A widely used technique for synthesizing nanocrystalline metals has been condensation of metal atoms from the vapor phase. In this technique, the metal is evaporated by heating and the evaporated atoms then cooled by exposure to an inert gas such as helium or argon to prevent chemical reactions, thereby enabling the purity of the metal to be maintained. The cooled atoms condense into single-crystal clusters with sizes typically in the range of 1 to 200 nm. The production of ceramic nanocrystals is similar, except that evaporated metal atoms are made to react with an appropriate gas, e.g., oxygen in the case of oxide ceramics, before they are allowed to condense. The resulting nano-sized particles may be compacted and sintered to form an article, often at a sintering temperature lower than that required for a microcrystalline powder of the same material. In any case, the sintering temperature must be kept sufficiently low to inhibit grain growth and retain the fine grained structure. While suitable for making powders and small compacted samples with excellent control over particle size, the condensation method is at present not practical for most applications other than experimental. A particularly limiting aspect of the condensation method is the difficulty of forming nanocrystalline materials of alloys because of the challenges associated with controlling the composition of the material from the vapor phase. Another limiting aspect of the condensation method is that high green densities in powder compacts are more difficult to achieve as a result of the nano-size particles produced. Furthermore, the nano-sized particles suffer from problems of agglomeration and oxidation due to their high surface area to volume ratio. Other methods that have been explored to synthesize nanocrystalline materials include aerosol, sol-gel, high-energy ball-milling, and hydrothermal processes. However, these techniques are limited in the range of alloys that can be produced, and have not produced nanocrystalline materials at a cost acceptable for practical applications.
From the above, it can be seen that more controllable and preferably low-cost approach are desired for synthesizing nanocrystalline solids for use in the manufacture of products. It is also desired to produce nanocrystalline solids from a wide variety of materials, including very hard materials and alloys that are difficult or impossible to process using prior art techniques. A solution to the above-noted shortcomings of the prior art is provided in commonly-assigned U.S. Pat. No. 6,706,324 to Chandrasekar et al., which discloses machining techniques for the large scale production of nanostructured (nanocrystalline and UFG) materials. According to Chandrasekar et al., whose teachings are incorporated herein by reference, large strain deformation during chip formation in machining leads to significant grain refinement and development of nanocrystalline and UFG microstructures in a wide variety of materials, including metals and alloys. The deformation that occurs in the shear plane of a chip can be seen in reference to FIG. 1, which represents the machining of a workpiece surface with a wedge-shaped indenter (tool). The material being removed by large strain deformation, namely, the chip, slides over the surface of the tool known as the rake face. The angle between the rake face of the tool and the normal to the work surface is known as the rake angle (α), which may be positive or negative as indicated in FIG. 1. The edge of the tool penetrating the workpiece is the cutting edge. The amount of interference between the tool and the workpiece is the undeformed chip thickness depth of cut (to) and the relative velocity between the tool and the workpiece is the cutting velocity (Vc). When the tool's cutting edge is perpendicular to the cutting velocity and the width of cut is large compared to, a state of plane strain deformation prevails, which is a preferred configuration for experimental and theoretical investigations of machining (though not a necessary condition for making nanostructured materials).
The chip formation in FIG. 1 is seen to occur by concentrated shear along a plane called the shear plane, where a shear strain (γ) is imposed during chip formation. The shear strain can be estimated by Equation (1) below:γ=cos α/sin φ cos(φ−α)  (Eq. 1)where the shear plane angle (φ) is a known function of to and tc. The effective Von Mises strain (ε) can be predicted usingε=γ/(3)1/2  (Eq. 2)Equation (1) shows that the shear strain (γ) can be varied over a wide range by varying the rake angle (α) from large positive to large negative values. Additionally, the friction at the tool-chip interface also affects shear strain (γ) via its effect on the shear plane angle φ.
In view of the above, Chandrasekar et al. teach that effective plastic strains in the range about 0.5 to about 10 and strain rates of up to 105 per second can be generated with appropriate machining conditions, as can a wide range of shear plane temperatures. These ranges of values are substantially greater than can be realized in typical severe plastic deformation processes. Geometric parameters of machining like depth of cut (to) rake angle (α) and cutting velocity (Vc) affect the shear deformation in a manner analogous to the action of dies in forging or extrusion. The effective plastic strain along the shear plane (deformation zone) in the chip can be systematically varied in the range of about 0.5 to about 10 by changing the tool rake angle, and to a lesser extent by changing the friction between tool and chip. The mean shear and normal stresses on the shear plane can be varied by changing the tool geometric parameters together with process parameters such as Vc and to, while the values of these stresses can be obtained from measurement of the forces. Finally, the temperature in the deformation zone can be systematically varied by changing the cutting velocity. For example, by cutting at very low velocities (about 0.5 mm/s), the temperature can be kept marginally above the ambient temperature while achieving very large strain deformation. Alternatively, temperatures where phase transformations (e.g., martensitic, melting) may be expected to occur in the chip can be realized by increasing the cutting velocity to higher values, for example, about 1 to about 2 m/s. The ability to change the friction along the tool-chip interface by a factor of up to three has also been demonstrated using low-frequency modulation of the tool-chip interface in conjunction with lubrication. The modulation assures that lubricant is always present at the interface between the tool and the chip. The extent to which friction (as well as the other parameters and conditions discussed above) can be controlled in a machining operation is not possible in other severe plastic deformation processes. In summary, the temperature, stress, strain, strain rate and velocity fields in the zone of deformation can be well estimated using available mechanics models or obtained by direct measurement. (See, for example, S. Lee, J. Hwang, M. Ravi Shankar, S. Chandrasekar and W. D. Compton, Metallurgical and Materials Transactions, Vol. 37A, 1633-1643, May 2006; M. Ravi Shankar, B. C. Rao, S. Lee, S. Chandrasekar, A. H. King and W. D. Compton, Acta Materialia, Vol. 54, 3691-3700, 2006.) Thus, very large strain deformation conditions can be imposed and varied systematically over a wide range, a range over and beyond that currently obtainable in other severe plastic deformation processes.
In view of the above, the teachings of Chandrasekar et al. provide a basis for producing material (such as continuous chips including ribbons, wires, filaments, etc., and discontinuous chips including particulates, platelets, etc.) having nanocrystalline and UFG microstructures in a wider group of materials and at lower costs compared to other processes capable of producing materials with nanocrystalline microstructures. For example, chips machined from 6061-T6 aluminum stock with a +5 degree rake tool have been produced to have generally equi-axed grains with a typical grain size of about 75 nm. As a result of the grain refinement achieved during machining, chips have been produced that exhibit hardnesses of about 150 HV, up to 50% harder than the original bulk stock, as reported in M. R. Shankar, S. Chandrasekar, A. H. King and W. D. Compton, Acta Materialia, Vol. 53, 4781-93, 2005. Such nanostructured chips can be consolidated into components or structures, for example, using powder metallurgy (PM) processes, powder extrusion, forging, spraying methods such as cold-spray, etc., as well as serve as important constituents in metal and polymer matrix composites. A further advancement based on Chandrasekar et al. is the capability of controllably producing nanostructured chips with a desired shape and size, as disclosed in commonly-assigned U.S. patent application Ser. No. 11/381,392 to Mann et al.
Notwithstanding the advancements achieved through the teachings of Chandrasekar et al., further capabilities in the production of articles having nanocrystalline and UFG microstructures are desirable.